Add conditional boss battle teams (variant teams by condition)

Wire up the existing condition_label column on boss_pokemon to support
variant teams throughout the UI. Boss battles can now have multiple team
configurations based on conditions (e.g., starter choice in Gen 1).

- Add condition_label to BossPokemonInput schema (frontend + backend bulk import)
- Rewrite BossTeamEditor with variant tabs (Default + named conditions)
- Add variant pill selector to BossDefeatModal team preview
- Add BossTeamPreview component to RunEncounters boss cards
- Fix MissingGreenlet error in set_boss_team via session.expunge_all()
- Fix PokemonSelector state bleed between tabs via composite React key
- Add Alembic migration for condition_label column

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/app/models/boss_pokemon.py

@@ -1,4 +1,4 @@
-from sqlalchemy import ForeignKey, SmallInteger
+from sqlalchemy import ForeignKey, SmallInteger, String
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.core.database import Base
@@ -14,6 +14,7 @@ class BossPokemon(Base):
     pokemon_id: Mapped[int] = mapped_column(ForeignKey("pokemon.id"), index=True)
     level: Mapped[int] = mapped_column(SmallInteger)
     order: Mapped[int] = mapped_column(SmallInteger)
+    condition_label: Mapped[str | None] = mapped_column(String(100))
 
     boss_battle: Mapped["BossBattle"] = relationship(back_populates="pokemon")
     pokemon: Mapped["Pokemon"] = relationship()